Alloys in which the solubility of at least one of the alloying elements decrease with decreasing temperature can be strengthened by age hardening. Age hardening is common to a number of alloying systems including magnesium alloys. The age hardening process in general involves three stages:
1) Solution heat treatment—in this stage an alloy is held at a very high temperature (close to the alloy solidus temperature) in order to obtain a single phase solid solution and to dissolve the alloying elements in the magnesium matrix.
2) Quenching—rapid cooling from the temperature of solution heat treatment using a quenching medium (such as cold water) in order to retain alloying elements in the solid solution and obtain a supersaturated solid solution.
3) Holding the as-quenched alloy at an intermediate temperature (artificial ageing) in order to promote the decomposition of the highly unstable supersaturated solid solution in which the alloying elements, often including the magnesium atoms, form precipitates throughout grains.
The strengthening during ageing generally occurs as a result of the formation of a fine dispersion of precipitates that reinforce the magnesium matrix and represent obstacles to movement of dislocations, thus increasing the alloy's ability to resist the deformation leading to failure. Generally, optimal strengthening is achieved in the presence of a high density of uniformly distributed and very closely spaced precipitates that cannot be easily bypassed by gliding dislocations.
Many cast and wrought magnesium alloys are age-hardenable. The most common are those based on the systems Mg—Zn(—Zr) (ZK series), Mg—Zn—Cu (ZC series), Mg—Zn-RE (ZE and EZ series; where RE means rare earth elements), Mg—Zn—Mn(—Al) (ZM series), Mg—Al—Zn(—Mn) (AZ and AM series), Mg—Y-RE(—Zr) (WE series), Mg—Ag-RE(—Zr) (QE and EQ series), Mg—Sn(—Zn, Al, Si) based alloys etc. In each system, magnesium typically comprises more than 85 weight %. Magnesium alloys containing Zn as the major alloying element are precipitation hardenable and comprise a great proportion of currently used magnesium alloys.
While the following description will focus on Mg—Zn alloys, it is to be understood that the invention is not limited to those alloy compostions and is applicable to all precipitation hardenable magnesium based alloys.
Heat treatable magnesium alloys are generally subjected to an elevated temperature heat treatment (commonly referred to in the art as “T6”) wherein the stage of artificial ageing (stage (3) of the age hardening process above) is conducted typically at a temperature between 150° C. and 350° C.
In the case of Mg—Zn alloys, the precipitation sequence above ˜110° C. has been reported to be:SSSS→(pre-β′)→β′1 rods ⊥{0001}Mg (possibly MgZn2)→β′2 discs ∥{0001}Mg (MgZn2)→β equilibrium phase (MgZn or Mg2Zn3)
The structure, composition and the stability of some of these phases have not yet been fully investigated and determined, however a number of reports agree that the maximal hardening due to the precipitation in Mg—Zn based alloys subjected to a conventional T6 heat treatment is associated with the formation of the rod-shaped transition β′1 phase. This phase forms perpendicular to the basal plane of Mg, possibly via another transition phase denoted pre-β′. On overageing, β′1 is replaced by a coarse β′2 phase in the form of a plate parallel to the Mg basal plane. The equilibrium β phase, MgZn or Mg2Zn3, may form upon high overageing. Precipitation at reduced temperatures (˜<110° C.) has not been clearly observed by transmission electron microscopy (TEM). While it is believed that GP zones may possibly form at reduced temperatures, the formation, structure, thermal stability and the sequence of the formation of GP zones have not yet been clarified.
Although many magnesium alloys undergo precipitation hardening, currently the most effective methods of increasing their mechanical properties preferably still include solid solution hardening, dispersion hardening and grain refinement. Even then, the tensile properties of most heat treatable magnesium alloys are limited compared to those of the currently used aluminum alloys, which is one of the main limitations for the wider application of magnesium alloys. Age hardening of magnesium alloys is generally not considered as being as effective in improving tensile properties as it is in the case of aluminum alloys. This is believed to be primarily because the number density of the precipitates formed during the conventional T6 ageing in magnesium alloys is several orders of magnitude lower than in the aged aluminum alloys. Therefore widely spaced precipitates that form in the T6 condition of magnesium alloys are easily bypassed by gliding dislocations and such alloys display reduced resistance to deformation.
Strengthening of magnesium alloys through age hardening would become more effective in the case of the formation of higher density of finely dispersed precipitates throughout the microstructure.
It would accordingly be desirable to make precipitation hardening more effective in increasing strength. This can then be used alone or in the combination with work hardening and grain refinement to increase the upper limit of the mechanical properties that can be achieved in magnesium alloys, thereby enabling wider and more competitive use of these light weight alloys. It would be particularly desirable to make precipitation strengthened magnesium alloys more ductile. It would also be desirable to improve those properties using an ageing process able to be conducted at lower temperatures than those of the conventional T6 ageing.
The present invention is based upon the surprising discovery by the inventor that age hardening of magnesium based alloys can be effected at significantly lower temperatures than are typically used during conventional T6 ageing, such as at ambient temperature. Moreover, the ageing response achievable using the invention can be comparable to or in some cases exceed, that achieved using conventional T6 ageing.
Age hardening at ambient temperature of any notable magnitude has never previously been observed in age-hardenable magnesium alloys, including the Mg—Zn based alloys, and it has been assumed that magnesium alloys therefore do not show any significant precipitation hardening response when held at reduced temperatures such as close to ambient temperature after quenching from the solution heat treatment temperature.